The prefix auto- means “self” or “by itself.” It comes from an ancient Greek word meaning “self,” and when you see it at the start of a word, it signals that something operates on its own or relates to oneself.

An automobile moves by itself (rather than being pulled by horses). An autobiography is a book someone writes about their own life. When a process is automatic, it happens by itself without someone controlling each step, like automatic doors that open when you walk near them.

You'll find auto- in many modern words. An autocrat is a ruler who governs by their own power alone. Autofocus cameras adjust their lenses by themselves. Autopilot systems fly planes without constant human control.

The prefix appears in technical and everyday language alike. Your body has an autonomic nervous system that controls breathing and heartbeat automatically, without you thinking about it.

Understanding auto- helps you decode unfamiliar words. If you see “autodidact,” you can guess it means someone who teaches themselves. If you hear about an “autoimmune” disease, you know it involves the body working against itself. Once you recognize this prefix, you'll spot it everywhere.